Ejector systems are used in the semiconductor industry for a variety of purposes. One use is in the area of automatically transferring a semiconductor substrate onto a work-holder for processing. For example, such an ejector system is used to automatically transfer substrates from their containers to wire bonding machines that make wire connections between electronic components attached on the substrate and electrical connection pads on the substrate.
More specifically, in a typical wire bonding machine, an ejector system is used to eject substrates (commonly in the form of leadframes) from a magazine onto a work-holder channel. The substrates are pushed one at a time onto the work-holder and held in position by clamping devices. Thereafter, a bonding tool performs wire bonding on the substrates on the work-holder while the substrates are being held by the clamping devices. An indexing system associated with the clamping device is responsible for precisely indexing the substrate and presenting it to the bonding tool. The bonding tool makes interconnections between the connection pads (such as leads) of the substrate and connection pads on the semiconductor device mounted on the substrate.
Conventionally, the ejector system comprises an ejector arm that is driven by a traction drive using a DC motor. A prior art ejector system 100 is illustrated in FIG. 1. The ejector system 100 has an ejector arm 102 to push a leadframe 104 into a work-holder channel (not shown). The ejector arm 102 is driven towards the leadframe 104 by frictional force from rubber rollers 106 pressing on the ejector arm 102. The rubber rollers 106 are mounted directly on a motor shaft 108 of a geared DC motor (not shown). Deep-grooved ball bearings 110 are positioned along the route of motion of the ejector arm 102 to provide smooth and precise linear motion to it. At least one of the ball bearings 110 is mounted on a preload arm 114. A spring 112 is coupled to the preload arm 114 to apply a certain preload on the preload arm 114 and the at least one ball bearing 110 to maintain steady and stable movement of the ejector arm 102. System initialization and motion control are achieved through a home sensor 116 and a limit sensor 118 positioned adjacent to the ejector arm 102 .
Due to variation of friction between the leadframe 104 and the magazine or misalignment between the leadframe 104 and the work-holder channel, it is possible for jamming of the leadframe 104 in the magazine or work-holder channel to occur. Thus, a jam detecting sensor 120 is placed just above the leadframe 104 to sense any lateral deformation of the leadframe 104 that would occur during jamming of the leadframe 104. Jam detection is important because semiconductor substrates are generally fragile and vulnerable, and excessive external forces acting on their components during a jam can result in damage.
There are certain drawbacks in this conventional design. Since it adopts a traction drive mechanism, its reliability and lifespan are limited by premature wear. Also, the thrust force is generally non-programmable and uncontrollable for different types of leadframes. A similar thrust force would therefore lead to different impacts on leadframes that comprise different materials, thicknesses and widths. Even with stringent control over manufacturing tolerance and assembly, force consistency is difficult to achieve for the above reason. Further, jam detection and protection capability depends considerably on the sensitivity of the jam detecting sensor 120. Especially for thick leadframes, the deformation in the leadframes may be small, making it very difficult to sense deformations in the leadframes if they are jammed. Finally, this jam sensing module entails significant additional cost, which is likely to be even higher than the cost of the ejector system itself.